In the production of moulds and cores, polyurethane-based binder systems are used in large amounts, in particular for mould and core production for the cold-box or polyurethane no-bake process.
Polyurethane-based binder systems for the cold-box and for the polyurethane no-bake process typically comprise two essential binder components, namely:
(1) a polyol component which comprises a binder having at least two OH groups per molecule and
(2) a polyisocyanate component which comprises a binder having at least two isocyanate groups per molecule.
These components are optionally solvent-containing and are usually packed and sold in separate containers.
Usually, the polyol component (first component) comprises a phenol resin having at least two OH groups per molecule. Of these, phenol resins of the benzyl ether resin type have become particularly important. These are the condensates of a phenol of the general formula I 
in which A, B and C are hydrogen, alkyl groups or alkoxy groups, with aldehydes of the general formula Rxe2x80x2CHO, in which Rxe2x80x2 is a hydrogen atom or an alkyl group having 1-8 carbon atoms. The reaction of phenols of the stated general formula with aldehydes of the general formula Rxe2x80x2CHO is carried out in the liquid phase, typically at a temperature below 130xc2x0 C. Catalytic amounts of ortho-directing divalent metal ions, such as Zn2+, are added to the reaction mixture.
Preferred benzyl ether resins correspond to the following general formula II: 
Here, R is hydrogen or a phenolic substituent in the ortho, meta or para position relative to the phenolic hydroxyl group; the sum of m and n is at least 2 and the ratio m/n is at least 1; X is hydrogen or CH2OH, the ratio of hydrogen to CH2OH being at least 1.
For use in a two-component binder system, phenol resins, in particular benzyl ether resins, are usually used as a solution in an organic solvent. The solvent is required for reducing the viscosity of the phenol resin for mixing with a moulding material and reacting with the polyisocyanate component.
The isocyanate component (second component) of the two-component binder system for the cold-box or polyurethane no-bake process usually comprises an aliphatic, cycloaliphatic or aromatic polyisocyanate having preferably between two and five isocyanate groups; mixtures of such polyisocyanates may also be used. Particularly suitable polyisocyanates among the aliphatic polyisocyanates are, for example, hexamethylene diisocyanate, particularly suitable ones among the alicyclic polyisocyanates are, for example, 4,4xe2x80x2-dicyclohexylmethane diisocyanate and particularly suitable ones among the aromatic polyisocyanates are, for example, 2,4xe2x80x2- and 2,6xe2x80x2-toluene diisocyanate, diphenylmethane diisocyanate and their dimethyl derivatives. Further examples of suitable polyisocyanates are 1,5-naphthalene diisocyanate, triphenylmethane triisocyanate, xylene diisocyanate and their methyl derivatives, polymethylenepolyphenyl isocyanates (polymeric MDI), etc. Although all polyisocyanates react with the phenol resin with formation of a crosslinked polymer structure, the aromatic polyisocyanates are preferred in practice. Diphenylmethane diisocyanate (MDI), triphenylmethane triisocyanate, polymethylene polyphenyl isocyanates (polymeric MDI) and mixtures thereof are particularly preferred.
The polyisocyanate is used in concentrations which are sufficient to effect curing of the phenol resin. In general, 10-500% by weight, preferably 20-300% by weight, based on the mass of (undiluted) phenol resin used, of polyisocyanate are employed. The polyisocyanate is used in liquid form; liquid polyisocyanate can be used in undiluted form, and solid or viscous polyisocyanates are used in the form of a solution in an organic solvent, it being possible for the solvent to account for up to 80% by weight of the polyisocyanate solution.
In choosing the solvents for the phenol resin component and optionally for the polyisocyanate component, it should be noted that although these do not participate in a relevant manner in the reaction between the isocyanate and the phenol resin in the presence of a catalyst, they may very well influence this reaction. One problem arises in particular from the situation that the two binder components phenol resin and polyisocyanate have substantially different polarities. This polarity difference between the polyisocyanate and the phenol resin limits the number of solvents which may be used to those which are compatible with both binder components. Such compatibility is necessary in order to achieve complete reaction and curing of a binder system. Although polar solvents of the protic and aprotic type are usually good solvents for the phenol resin, they are not very suitable for the polyisocyanate. Aromatic solvents in turn are compatible with polyisocyanates but are not very suitable for phenol resins.
In practice, mixtures of polar and nonpolar, aromatic-containing solvents which are tailored to the respective binder system (phenol resin and polyisocyanate) are therefore usually used. Moreover, the individual components of the solvent mixture should not have too low a boiling range, so that the solvent cannot become ineffective too rapidly as the result of evaporation.
Nonpolar, aromatic-containing solvents used to date are preferably mixtures of high-boiling aromatic hydrocarbons, i.e. mixtures of aromatic hydrocarbons having a boiling range above 150xc2x0 C. at atmospheric pressure. Polar solvents which have been used are, inter alia, specific sufficiently high-boiling esters, such as, for example, the xe2x80x9csymmetricalxe2x80x9d esters which are described in German Patent 27 59 262 and in which both the acid radical and the alcohol radical have a relatively large number of C atoms (about 6-13 C atoms) in the same range.
With all advantages of the polyurethane binder for casting technology, it is still felt to be a disadvantage that excessively high benzene emissions occur during pouring of a casting in a mould which comprises a binder based on a polyurethane. These benzene emissions during pouring, but also evaporation and devolatilization prior to the pouring, constitute considerable workplace pollution which generally cannot be trapped by protective measures, such as extractor hoods or the like.
It was therefore the object of the present invention to provide a polyurethane-based binder system for the cold-box and for the polyurethane no-bake process, which binder releases only small amounts of aromatic compounds during foundry operation.
According to the invention, this object is achieved by providing a two-component binder system consisting of a phenol resin component and a polyisocyanate component, the phenol resin component comprising a phenol resin having at least two OH groups per molecule and the polyisocyanate component comprising a polyisocyanate having at least two isocyanate groups per molecule, and at least the phenol resin component containing a solvent, wherein
the solvent for the phenol resin comprises a substance which is selected from the group which comprises alkyl silicates, alkyl silicate oligomers and mixtures thereof (i.e. mixtures of different alkyl silicates, mixtures of different oligomers and mixtures of alkyl silicate(s) with oligomer(s) and/or
the polyisocyanate component contains a solvent which comprises such a substance, i.e. a substance which is selected from the group which comprises alkyl silicates, alkyl silicate oligomers and mixtures thereof.
The amount of alkyl silicates, alkyl silicate oligomers and mixtures thereof in the phenol resin component is advantageously in the range between 1 and 40% by weight.
The amount of alkyl silicates, alkyl silicate oligomers or mixtures thereof in the polyisocyanate component (if this requires a solvent) is advantageously likewise in the range between 1 and 40% by weight.
The invention is based on the surprising discovery that alkyl silicates, i.e. alkyl esters of silicic acid, can be used as a solvent or as a solvent component (firstly) for cold-box phenol resins and polyurethane no-bake phenol resins and/or (secondly) for the polyisocyanates used in the cold-box or polyurethane no-bake process, without there being any disadvantages.
Oligomers of alkyl silicates, e.g. oligomers of tetraalkyl silicates, such as Dynasil 40 (Degussa-Hxc3xcls; CAS: 68412-37-3), oligomers of alkyltrialkoxysilanes, oligomers of dialkyldialkoxysilanes and oligomers of trialkylmonoalkoxysilanes can be used in just such a manner, in particular for the phenol resin component.
The phenol resin and the polyisocyanate can be selected from the group consisting of the compounds usually used in the cold-box process or the no-bake process. However, the compounds and groups of compounds mentioned further above are preferred.
It is preferable in particular if the phenol resin component comprises a phenol resin of the benzyl ether type, as described above with reference to the general formula II. When alkyl silicate oligomers are used, it is expedient in individual cases to use an alkylphenol, such as o-cresol, p-nonylphenol or p-tert-butylphenol, in the mixture, in particular with phenol, for the preparation of the phenol resin.
The polyisocyanate component preferably comprises polymeric diphenylmethane diisocyanate (polymeric MDI), advantageously more than half the isocyanate groups in the polyisocyanate component being assigned to the diphenylmethane diisocyanate molecules.
In the group consisting of the alkyl silicates, the tetraalkyl silicates and in particular the tetraethyl silicate have proved particularly suitable solvents. Tetraalkyl silicates, such as tetraethyl silicate, can be used as solvents in the phenol resin component and/or in the polyisocyanate component of a binder system.
Alkyl silicates or alkyl silicate oligomers, in particular tetraalkyl silicates, such as tetraethyl silicate, or corresponding mixtures of alkyl silicate(s) with/or alkyl silicate oligomer(s) can be used together with cosolvents as solvents for the polyisocyanate component; the mass ratio of alkyl silicate (or oligomer or mixture) to cosolvent is usually greater than 1:50, preferably greater than 1:4. In the case of mass ratios of less than 1:50, the presence of the alkyl silicates (or oligomers or corresponding mixtures) in the solvent for the polyisocyanate component has only little effect on the behaviour of the binder system.
Advantageously, it is possible to use an alkyl silicate, an alkyl silicate oligomer or a mixture of alkyl silicate(s) with/or alkyl silicate oligomer(s) as the sole solvent or predominant solvent component (mass ratio of alkyl silicate to cosolvent  greater than 1:1) for the polyisocyanate (for example, polymeric MDI).
In particular, tetraethyl silicate or a mixture of tetraethyl silicate with other alkyl silicates or with alkyl silicate oligomers can be used as the sole solvent or predominant solvent component (mass ratio of tetraethyl silicate or mixture to cosolvent  greater than 1:1) for the polyisocyanate.
If tetraethyl silicate is used as the sole solvent or predominant solvent component for the polyisocyanate, the mass ratio of polyisocyanate to tetraethyl silicate in the polyisocyanate component should be in the range between 95:5 and 65:35.
The solvent for the phenol resin preferably consists of
(a) alkyl silicate, alkyl silicate oligomer or a corresponding mixture of alkyl silicate(s) with/or alkyl silicate oligomer(s) and
(b) a cosolvent.
The mass ratio of alkyl silicate (or oligomer or mixture) to cosolvent may vary within wide limits and is usually between 1:60 and 5:1, preferably between 1:44 and 35:10. A preferred alkyl silicate in turn is tetraethyl silicate.
Additives increasing the polarity of the solvent are preferably used as cosolvent for the phenol resin. Numerous polar compounds are suitable for this purpose, for example a mixture of dimethyl esters of C4-C6-dicarboxylic acids, referred to as xe2x80x9cdibasic esterxe2x80x9d or xe2x80x9cDBExe2x80x9d for short.
Alternatively, the methyl monoesters of one or more fatty acids having a carbon chain from 12 C atoms, described in our own EP 0 771 559 A 1, can be used as cosolvent, for example rapeseed oil methyl ester.
Also as an alternative, every other solvent customary for phenol resin component of a two-component binder system can also be used as cosolvent in addition to alkyl silicate, alkyl silicate oligomer or a corresponding mixture. The person skilled in the art can determine the suitable mixing ratios in the specific case on the basis of a few preliminary experiments.
Although the use of aromatic compounds as cosolvents for the phenol resin (or polyisocyanate) component is not ruled out in principle, for ecological reasons it is clearly preferable completely to dispense with aromatic compounds in the solvents for the phenol resin and the polyisocyanate component. For the use of the alkyl silicate-containing solvents according to the invention, this is possible without disadvantages in the production of moulds and cores and in the casting thereof. This is to be regarded as substantial progress compared with the binder systems used to date in practice.
The use of alkyl silicates (such as tetraalkyl silicate) or alkyl silicate oligomers as a solvent (or solvent component) for the phenol resin and/or polyisocyanate component of a two-component binder system for the cold-box and for the polyurethane no-bake process is advantageous not only from the ecological point of view. From the technological point of view, too, the use of alkyl silicates or alkyl silicate oligomers is beneficial. In particular, the thermal stability of moulds and cores in the production of which binder systems according to the invention were used is particularly high. In addition, such moulds and cores are distinguished by lower gas pressure generation compared with conventional moulds and cores during casting.
Particularly high strengths are obtained if tetraethyl silicate is the main component of the solvent for the phenol resin and, if necessary, the sole solvent for the polyisocyanate component. Based on the solvent used altogether, the amount of alkyl silicate (or tetraalkyl silicate), alkyl silicate oligomer or corresponding mixtures should however exceed the amount of aromatic solvent components in every case.
A number of particularly preferred (and optionally substituted) alkyl (ortho)silicates are shown in the Table below, beginning with the preferred tetraalkyl silicates:
Table 1
Tetraalkyl silicates: Tetraethyl (ortho)silicate; tetra-n-propyl silicate
Trialkyl silicates: Triethyl silicate; trialkyl silicates (in particular triethyl silicates) having an aryl function on the fourth oxygen atom (Sixe2x80x94Oxe2x80x94Ar; Ar=aryl radical)
Dialkyl silicates: Diethyl silicate; dialkyl silicates having an aryl function on the third and/or fourth oxygen atom (Sixe2x80x94Oxe2x80x94Ar)
Monoalkyl silicates: Monoethyl silicate; monoalkyl silicates having an aryl function on the second and/or third and/or fourth oxygen atom (Sixe2x80x94Oxe2x80x94Ar)
Substituted silicates:
a) Aryl- or alkylalkoxysilanes, i.e. compounds of the type R1n=1-3Si(OR2)m=4xe2x88x92n with R1=alkyl or aryl radical and R2 =alkyl radical; e.g. dimethyldimethoxysilane (R1=CH3; n=2; R2=CH3, m=4xe2x88x92n=2);
b) organofunctional silanes, i.e. compounds of the type R1n=1-3Si(OR2)m=4xe2x88x92n with R1=functional group, such as 3-aminopropyl or 3-ureidopropyl or 3-glycidyloxypropyl and R2=alkyl radical; e.g. 3-aminopropyltriethoxysilane, 3-ureidopropyltriethoxysilane or 3-glycidyloxypropyltrimethoxysilane.